Process of digesting pulp and apparatus therefor



, April-25,1933.

R. H. MOKEE PI QOCESS ,OF DIGESTING PULP AND APPARATUS THEREFOR 2 Sheets-Sheet 1 Filed May 4; 1929 TTOR EY April 25,1933.

' R. H. M KEE PROCESS OF DIGESTING PULP AND APPARATUS THEREFOR Filed May 4, 1929 2 Sheets-Sheet 2 A TTORNEY.

tom of steam was used,

have been made Patented Apr. 25, 1933 OFFICE RALPH H. McKEE, or JERSEY CITY, NEWLJEBSEY PROCESS OF DIGFSTING PULP APPARATUS THEREFOR Application filed May 4,

The present invention relates to the art of digesting pulp, and more particularly'to an improved process of digesting pulp and an apparatus therefor.

It has been the customary practice heretofore to cook wood chips in a digester provided with a central, single steaminlet which was located adjacent to the discharge outlet at the bottom of the digester. With this 0 arrangement, the injected steam with heated cooking liquor passed up through the central portion of the digester to the top thereof, then outwardly toward the periphery of the digester, and finally downwardly to the botthe digester. This circulation of cooking liquor continued as long as steam was injected into the mass of wood chips and cooking liquor. The temperature, as is well known, was highest in the center of the di- 20 gester and was lowest in the region adj acent the periphery thereof. As the cooking liquor descended in the peripheral regions of the digester, the liquor was cooled still more due to radiation and conduction heat losses.

This tended to accentuate the temperature difierences within the mass of digesting pulp and to efiect a detrimental non-uniformity in operatingconditions. Due to the nonuniformity of thermal conditions there was 0 a tendency for the digester to become overheated to such an extent that the acid of the cooking liquor was decomposed and that a portion of the wood chips become brown and remained raw. When superheated the non-uniformity and overheating within the digester were accentuated and the yield of good pulp was lower and the amount of screenings, due to brown and raw chips, was higher. Many attempts to remedy the aforesaid disadvantages, particularly and decomposition of t e acid of the cooking liquor with their consequent detrimental results, butnone have, as far as I am aware, satisfactorily and successfully rovided a commercially practical and efficient process and apparatus.. The suggestions thusfar made concerned themselves with attempts to modify the standard method and toadhere substantially to the centrally heated circular-1 local overheating 1929. Serial No. 360,341.

tion of liquor in either an upward or a downward direction and with either internal or external heating rather than any radical or fundamental departure from the standard digesting process.

In accordance with my discovery, however, I contemplate a basic departure from the standard pulp digesting method and the provision of a process which overcomes fundamental defects inherent in the standard method. It is also within the contemplation of my discovery to provide a processof digesting pulp which gives a. practically even distribution of heat throughout the-di esters and a uniformity ing liquor onthe mass of wood .chi s and which has practically-no danger 0 localoverheating and avoids dead cold spaces with resultant uncooked chips. A further object of my invention is 'to provide a pulp digesting process which permits keeping more sulfur dioxidein solution in the cooking liquor due to the maintenance 'of an average low temperature and which effects a thorough penetration of wood chips before the critical temperature of about 120 C. is reached. Another object of my invention is to provide a process which prevents the tendency towardthe formation of excessive temperatures that drive ofi too much SO with a concomitant dilution of the acid in the cooking liquor toward the terminal period of the digestion of chips, which process has very little tendency to liberate gases and to cause shocks and noises in the di-' gester and which is simple, economical and.

- practical.

Other features and objects of my invention will become apparent from the following description taken in conjunction with th accompanying drawings, in 'which:

of chemical action 0 cook- Fig. 1 is an elevational view, partly in section, of my improved digester used for Yearrying my new process into practice;

Fig. 2 is a sectional view taken on the line 2 20f Fig.1; g 1 Fig. 3 is a view similar to Fi 1 of a modifie'd digesteremploying electrical .heat';

. Fig. 4 illustrates diagrammatically a di- 100] line.

gester provided with my new circulation system; and

Figs. 5 and 6 are diagrammatic views illustrating the circulating systems heretofore used in prior digesters.

Referring more particularly to Fig. 1, the numeral 1 designates-the shell of my improved pulp digester. The shell preferably consists of a main cylindrical intermediate portion 2 and a conical bottom and top 3, 3. Any suitable construction'may be employed for the shell but I prefer to construct it of riveted or welded plates of a material resistant to the action of sulfite cooking liquor. A material of this type, for example, is described in my Patent No. 1,841,? 52, relating to Industrial chemical vessels issued on J anuary 19, 1932. This material consists of an alloy of iron containing about 15% to 20% chromium'and about 7.5% to 10% nickel. For instance, an alloy steel containing about 18% chromium, about 8% nickel, say under 0.20% carbon, about 0.05% manganese and 0.50% silicon, and the remainder iron is a suitable material.

The conical top 3 is preferably provided with a neck 4 which is flared into a flat annular mouth 5. Seated upon mouth 5 is a cover 6 which is bolted thereto by means of a plurality of swing bolts 7. hese bolts may be of a suitable type, as is well known in the art. In my improved digester I prefer to employ a solid cover which does-not have the customary outlet pipes and the like incorporated therein, which can be easily made from the material referred to hereinabove and which can be conveniently handled in daily operations and use.

In neck 4 or conical top 3 a well-known relief line 8 containing valve 8' is prov vided in an appropriate manner. The inlet port 9 within the digester is protected by a strainer 10 having perforations, say about in size. A second relief line 11, commonl known as the side relief, contains a suita le valve 12 and projects from the upper part of the main cylindrical portion 2 as may be seen from Fig. 1. A strainer 13 having a construction similar to strainer 10 is secured over inlet 14 of the side relief The pu ose and function of these valves are well own to those skilled in the art and, consequently, no detailed description is necessary.

The shell 1 is preferably supported by a plurality of legs or standards 15 herein shown as six in number. Other suitable supporting means, however, may be employed for mounting the digester shell as one skilled in the art will readily understand. Mounted around the upper portion of the conical bottom 3 and preferably above the standards 15 is a steam feed pipe 16 which supplies a plurality of steam jets 17 within the digester shellwith steam. These steam jets are preferably located adjacent to the periphery of the bottom of the main cylinder 2 and are pointed upwardly. Any appropriate number of steam jets may be used but I prefer to employ three or more and I have illustrated a digester with six steam jets. A valve 18 may be provided for each steam jet and/or a main valve 19 may be incorporated in the main steam line 20.

The base of the conical bottom 3 termi nates in an outlet port 21 to which is secured an outlet fitting 22, such as a right angle pipe section, by a plurality of bolts, for instance, or other suitable means. Associated with the outlet fitting is a customary blowoff valve 23 for controlling the discharge of the contents from the digester to outlet line 24 which is directed at a well known target (not shown) as is usual in the sulfite pulp art. When the digester is made of my alloy steel referred to hereinabove or of a metal having similar properties, I prefer to make the conical bottom 3 shallow in contradistinction to the steep bottoms of conventional digesters. The angle of the conical bottom from the horizontal may vary depending upon the particular circumstances but an angle of, say, 30 is satisfactory. Ordinarily the angle of conventional digesters is about 7 0. With my improved digester there will be a saving of material and of labor and due to its smooth inner surface there will be less loss in blowing (i. e. in emptying the digester) due to the bridgin or adherence of solid material to the surface of the digester.

My improved pulp digester is provided with the customary and conventional auxiliaries. For instance, a sampling cock 25 to draw off samples of cooking liquor for analysis is shown mounted at an intermediate point on the side of the main cylinder 2. Positioned above the sampling cock 25 is the usual thermometer well 26 which is intended for the reception of a thermometer for measuring the temperatures of cooking liquor within the digester. In view of the fact that other auxiliaries are well known, it

is not necessary to illustrate them. For instance, external insulation may be used, as it is in some cases, to lag the outside of the digester so as to prevent the loss of excessive quantities of heat. I

A modification of my improved digester is illustrated in Fig. 3. The entire construction of the modified digester is the same as that described hereinabove with the exception that electrical means are employed to furnish the heat for the modified digester instead of steam jets. It will not be necessary to repeat the description of the digester depicted in Figs. 1 and 2.

The electrical heating arrangement provided for digester 1 may be of an appropriate type. For instance, an electrical resister heating device may be applied to" the digester or an electrical induction heating device may be used. Either of these types, as those skilled in the art will readily understand, can be applied to a digester with all of the necessary auxiliaries. I prefer to employ the electrical induction heating for a variety of reasons and I have illustrated this type in Fig. 3. u

An annular metal shelf supported by a plurality .of strong brackets 31 is afiixed, preferably by welding, to'the lower portion of the main cylindrical shell 2ladjacent to conical bottom 3. Upon this shelf a wall 32 of electrical insulating material is built up to a suitable height, say fourfeet or so. Any suitable electrical insulating material may be employed but I prefer to use carborundum (silicon carbide) bricks. The wall 32 is preferably built up of alternate rows 33 and 34 which are made of bricks laid on their fiat side and bricks laid on their edge as one will readily understand. By laying the bricks in this manner channels 36 are formed between adjacent rows 33. In order to provide a connection between adjacent channels 36 one or more bricks are broken or cut oil as indicated by openings 35, so that they do not project be yond the face of the bricks in rows 34. An electrical conductor 37 can be wound in channels 36 so as to extend from the bottom of wall 32 to thetop thereof. The ends 38 and 39 of conductor 37 can be secured in binding posts 40 and ll-respectively. These binding posts act as termlnals of the electrical conductor to which a source of alternating electric current can be connected. Of course, the source of current is provided with well-known auxiliaries for the supplyof proper and controlled electrical energy.

The total number of turns of conductor 37 on wall 32 and the number of turns in each channel will vary with the particular circumstances as those skilled in the art of electric induction furnaces willreadily understand. For instance, the number of turns will vary according to the voltage of the source of alternating current feeding the conductor 37 (.i. e. to the electrical current which the voltage forces through the conductor), to the heat required by the cooking liquor and the mass of woodchips within the digester, to the size of the digester, to the rate and temperatures at which the heat ing is to be conducted and to other related factors. Then again in the preferred embodiment conductor 37 is made as a hollow pipe through which cooling water may be passed.

When alternating electric current is supplied to conductor 36 induced currents are set up in the steel shell of the digester. These induced currents, as is well known, cause the evolution of heat. In'view of the fact that this heat is efiective within the walls of the digester shell it is transmitted V er, it rises upwardly to the upper portion of the body of cooking liquor where it then moves toward the central area thereof Upon reaching the central area it moves downwardly and takes the path as indicated by the arrows within the digester in Fig. 3. This circulation continues as long as heat is applied to the electrical heating system and functions in the same manner as in my steamheateddigester, all as will be more fully described hereinafter.

When electrical resistance heating is to be employed, conductor 37 may be made, for instance, of the well known resistance alloy known as nichrome and described in U. S.

Patent Number 811,859 to Albert W. Marsh.

With the passage of electrical current through nichrome conductor 37 heat is generated within the conductor and is given off to the surrounding environment. Since bricks made of silicon carbide are good thermal conductors, the heat generated in the nichrome conductor is readily transmitted to the shell of the digester and the contents thereof. In this manner the 'cook ing liquor within the digester can be conveniently brought to the desired temperatures wlthin the predetermined eriod of time as set forth hereinafter. (If course, the .outside of wall 32 may be lagged with an insulating brick made of infusorial earth or the like so as to conserve heat and prevent its loss by dissipation to the atmosphere.

In carrying my improved process of digesting pulp into practice I prefer to use the apparatus described hereinabove although any suitable pulp digester may be employed. A typical run embodying my process may be described in connection with my digester.

illustrated in Figs. 1 and 2 as follows: The digester is first filledwith wood chips and a cooking acid or liquor carrying about 4% of free sulfur dioxide .and 1% of combined sulfur dioxide (calculated as CaSO is run into the digester until the chips are about covered. by the cooking liquor.- The digester is closed with its cover 6 and proper valve connections are made via valves 18 and 19 to permit a flow of steam from the main steam line 20 to the peripheral steam jets 17 and into the cooking liquor contained in the digester. Steam is preferably turned on at such a rate that the digester will reach a temperature of 115 C. in about twohours. After atemperature of 115 C. has been reached, steam is then slowly injected via the peripheral jets until a temperature of about 145 C. ,is reached. This will take about six additional hours and will be accompanied -by relieving the digester in the well known manner from time to time by means of the top relief line 8. Sometimes it will be desirable and necessary to use the side relief line 11 from the digester in case the temperature rises too rapidly. When the temperature has reached 145 C. the steam is almost turned off and the cooking continued at this temperature for about one and a half hours longer. This is accompanied by relief of thedry gas, as is customary in pulp digestion. During the last hour the pressure will gradually decrease and at the end of the period when the pressure gauge shows a pressure of only about twenty to thirty pounds above atmospheric pressure, the bottom valve 23 is opened and the contents of the digester blown into the blowpit.

The circulation of cooking liquor within a pulp digester when proceeding in accordance with my process may be clearly observed from Fig. 4 wherein a pulp digester is diagrammatically illustrated. Steam entering the digester via the peripheral steam pulp, i. e.

During this upward. flow tions are m'aintaine jets causes a substantially =vertical upward flow of cooking liquor contiguous to the inner periphery of the digester shall to the upper portion of the body of cooking liquor.

the liquor is being heated by the \steam coming from the steam jets so that the eripheral portion of the cooking mass is s ightly hotter than the central portion. At the upper portion of .the digester the cooking liquor flows toward the center of the digester and then downwardly through the central portion of the digester to the bottom cone thereof. The cooking liquor thereafter flows upwardly alon the walls of the digester as indicated by the arrows in Fig. 4. With circulation of the aforesaid type it is possible to maintain temperature conditions substantially uniform throughout the body of the cooking liquor. I

In utilizing my rocess thermal condisubstantially uniform through the digester and the contents thereof can be heated to, say,'abo'ut 100 C. without any portion becoming overheated locally to, say, about 120 C. or without any spots or portions remaining cold. By effecting a uniform heating a-thorough penetration of the wood chips by the cooking liquor can be obtained. After the penetration of the chips with cooking liquor, which occurs in my process at about 100 C. within about the first two hours, the entire contents of the digester can be heated to-a cooking-temperature of about-145 C. By maintaining "the cooking temperature for a suitable period of, say, about six hours, is thorough] cooked withouthavm burnt arkened and undigeste chips.

the entire charge:

i p ifip containing In this manner I am able to obtain a maximum yield of good pulpand a minimum amount of burnt pulp.

In the digesting of pulp it is now believed that weak acid salt, such a calcium bisulfite, when the temperature is high (say about 120 C. orabove), is largely hydrolyzed to give free lime (Ca(OH) and sulfur dioxide (S0 which is in solution and not sulfurous acid (H SO When this occurs.

before penetration of the wood chips, the cooking acid is decomposed and the chips are burnt andcome out of the digester brown in colorand are discarded when the pul is screened. These disadvantages are avoi ed by the use of my process. It is my belief that in digesting wood chips in accordance with my inventionthe calcium bisulfite solution containing sulfurous acid has ample opportunity to wet and to diffuse into the fibers of the wood chips before the temperature for chemical action is reached. Under usual conditions this temperature is .about 120 C.

The circulation of the conventional method of digesting pulp heretofore employed is illustrated dia rammatically in Fig. Steam was usually injected via a single central jet located in' thebottom cone or via several jets located within the central region so that a vertically upward flow of cooking liquor was effected at the center of the digester. Upon reaching the top of the cooking liquor, the flow would be directed towards the periphery of the di ester and would then pass do nwardly a jacent to the walls of the digester to the bottom of the cone where the rising steam would carry the cooking liquor upwardly again as may be clearly seen from the arrows in Fig. 5. With this type of circulation the center of the digester was hottest and the outside was coldest.

Due to the. large surface on the outside of the digester, the heat i loss was increased and the difference in tem rature between the peripheral portions of tiii e body of cooking liquor. and the central ortion was accentuated. For example, di erences of temperature of about C. have been observed even in the second hour of heating.

During later periods of a run differences of temperature of about 15 to 20 C. were quite common. Ordinarily the center of the digester was 10 C. or more hotter than ortions of the periphery, even whenhan led by the most experienced operators. With non-uniformity of thermal conditions within the digester, some portions thereof wereoverheated andin somecases a tem rature of 120 C. was reached before a thbrough penetration of the wood chips occurred. Due to the local overheating before the .whole digester reached a condition where the wood chips were'thoroughl penetrated by cookliquor, burnt up was produced, ei. e. arkened-ami undigested am aware, none have been satisfactory or successful. A typical example of one of the proposed modified methods is diagrammatically illustrated in Fig. 6. Instead of injecting steam directly into the digester to cause the circulation a centrifugal pump C was used to cause a forced circulation of the cooking liquor through the digester as depicted by the arrows in Fig. 6. In some cases the cooking liquor was heated in an external heater H instead of by direct injection of steam into the body of the cooking liquor. In either case there was a tendency for the central portions to be hotter than the peripheral portions of the digester and a non-uniformity of thermal conditions was present within the digester.

With my improved method of digesting pulp involving heating by a plurality of peripherally-located steam jets, the contents of the digester are far more evenly heated and the thermal conditions within the digester are practically uniform. The difi'erences of temperature with ,my system of heating and circulating will be much less than in prior methods noted hereinabove and will be, for instance, of the order of, say A of those mentioned heretofore. My use of steam injection at a plurality of peripherally-located pjoints not only obviates the danger of local overheating but also avoids having dead cold spaces with resultant uncooked chips. More sulfur dioxidecan be kept in solution withniy method due to the uniformity of temperature and the maintenance of an average low temperature.

It is to be observed that by the use of my digester provided with an electrical heater, it is possible to obtain in a convenient manner a good control over the heating of the cooking liquor and to effect a peripherally upward and a centrally downward circulation of the cooking liquor. As no steam is introduced into the body of cooking liquor with electrical heat, there is no tendency to dilute the cooking liquor during the run with a consequent weakening of the acid in the liquor. The strength of the cooking liquor is consequently protected against detrimental dilution throughout the run.

While I have described the process as exclusively for making sulfite pulp it is "to be observed that my method is equally applicableto pulp made by the soda process .or by the kraft process (sulfate process).

Although certain specific embodiments of understood that the invention is not restricted to such particular embodiments. For instance, the electrical heatingmay be replaced by a jacket for heating by oil or other heated fluid. Various other modifications in detail and in the arrangement may be made by those skilled in the art without departing from the invention as defined in the claims.

' What I claim is: I I

1. A pulp digester having electrical heating means. j

2. A pulp digester having a main body portion and a top and bottom, and electrical heating means mounted around the lower portion of said main body portion and adapted-to heat cooking liquor Within said digester at the outermost peripheral portions of said digester whereby an upward flow of cooking liquor adjacent to said outermost peripheral portions is efiected."

3. A pulp digester having a metallic shell free from an inner lining and constituted of ,an alloy steel containing about 7.5 to 10% nickel and about 15% to 20% chromiumand of a main body portion and a top and bot tom, and having an electrical heating means mounted around the lower portion of said shell whereby heat is transmitted through said metallic shell directly to cooking liquor.

4. A pulp digester comprising a main cylindrical body, a top provided with a charging mouth and cover and a relief valve; a

shallow conical bottom devoid of a strainer; a free, unobstructed outlet located in the central region of said bottom and capable of permitting the discharge of cooked pulp by blowing; a plurality of steam jets mounted adjacent to the inner surface of said cylindrical body in direct contact with chips and lindrical body composed of an alloy, nickel-- chromium steel and having the inner surface thereof free from lining material, a top made of said alloy steel and provided with a charging mouth and cover. and a relief valve; a shallow conical bottom devoid of a strainer and made of said alloy steel, said conical bottom having an angle of approximately 30 a free, unobstructed outlet located in the central region of said bottom and capable of permitting the discharge of cooked pulp by blowing; a plurality of steam jets mounted adjacent to the inner surface of said cylindrical bodyin direct contact cooking liquor in the digester and not lower Y mum speed of How of cooking liquor contiguous to the inner surface of said cylindrical body; and valve means to control steam flowing to said jets.

6. A pulp digester comprising a main cylindrical body, a top provided with a charging mouth and cover and a relief valve; a shallow conical bottom devoid of a strainer; a free, unobstructed outlet located in the central region of said bottom and capable of permitting the discharge of cooked pulp by blowing; heated means provided adjacent to a the cylindrical body and located not lower than the region adjacent to the junction of the cylindrical body and the conical bottom, said heating means being in direct contact with chips and cooking liquor in the digestor and being arranged to heat cooking liquor in a region adjacent to the inner surface to a temperature higher than any other'region within the digestor and to give to said cooking liquor in the region contiguous to the inner periphery of said cylindrical body a speed of flow in an upward vertical direction higher than any other region within said digester whereby an upward flow of cooking liquor only takes place adjacent to j the inner periphery of the cylindrical body and a downward flow only occurs in the central portion of the digester; and means to control said heating means.

7. The process of digesting pulp in a digester having a cylindrlcal body and a conical top and bottom which comprises charging an unobstructed space in a digester with wood chips and cooking liquor, region closely adjacent to the inner periphery of the cylindrical body to give to said cooking liquor in said region a higher temperature and a greater speed of upward, vertical flow than any other region within said digester, limiting the upward flow of cooking liquor in a region adacent to the inner periphery of said cylindrical body and a downward flow occurs in the central portion of said digester, maintaining the cooking liquor and the wood chips adjacent to the inner periphery ofsaid body hotter than the chips and liquor in the central portion,

continuing the heatmg until the pulp is digested and then discharging the cooked pulp by blowing. v

8. The process of digesting pulp in a digester having a cylindrical body and a coni cal top and bottom which comprises charging an unobstructed space'in a digester with .wood chips and cooking liquor, heating a region closely ad'acent to the inner penph: cry of the cylin heating a ical body to give to and cooking liquor in said region a higher temperature and a greater speed of upward,

central portion of said digester, maintaining the cooking liquor and the wood chips adjacent to the inner periphery of said body gester, limited the u ward hotter than the chips and liquor in the central portion, continuing the heating until the pulp is digested and then discharging the cooked pulp by blowing.

In testimony whereof I have hereunto set my hand.

. RALPH H. MoKEE.

CEitTIFICATE or'eoRRrcnoN.

- Patent No. l,%5,'73i. April 25, 1933.

' I RALPH H. McKEE.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction, as follows: Page 2, line 130, for "resister" read resistor; page 4, line 29, for "shall"- read shell; line 70, after "that" insert the article "a" and in the same line for "a" read as; page 6. line l8, claim 6, tor "heated" read heating; and line 72, claim 8, for "limited" read limiting: and that the said Letters iatent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of November, A. D. l933.'-,

F. Hopkins l Acting Commissioner oirPatents. 

